<html>
    <meta charset="utf-8"/>
    <body>
    <form action="https://www.baidu.com/"onsubmit="return test()">
        <input type="text" id="uname"/>
        <input type="submit" value="提交"/>
    </form>
    </body>
    <script>
        function test(){
            var input_obj = document.getElementById("uname");
            if(input_obj.value){
                return true;
            }else{
                alert("输入用户名")
                return false;
            }
        }
    </script>
</html>